1. Field of the Invention
The invention is directed to a control system and a control method for injection molding machines.
2. Description of Related Art
Controlling injection molding machines requires to perform as exact a control of the speed and/or position as possible. Such controls are necessary, for example, for displacing the closing unit, displacing and driving the injection unit, and the like. Control systems for injection molding machines use PID (proportional plus integral plus derivative) controllers for control purposes. Speed is controlled, for example, by a speed profile being inputted into a memory means by a user. The user profile thus represents the target values. By means of the control system, for example, the current actual value of the speed is compared to the required target speed value. Depending on the current deviation between the target value and the actual value, the PID controller determines a control variable in order to achieve an adjustment of the actual value to the target value. This method is well suited, for example, for controlling constant speeds, e.g. the rotation of a feed screw, since the speeds are to be maintained constant over a longer period of time. However, if accelerations are to be controlled, the known control system has a drawback that the PID controller responds exclusively to a current deviation between the target value and the actual value, whereby the actual value always lags behind the target value.
A target value of a speed is predetermined as a target value development 10 (FIG. 1) and has a trapezoidal development. In a corresponding speed control using a PID controller of the prior art, an actual value follows the development 12 shown as a dotted line. Depending on a current comparison between the actual value 12 and the target value 10, a development of a control variable 14 shown in a dashed line is calculated, and a corresponding control variable is transmitted to the drive motor, a corresponding control or the like. As is obvious from FIG. 1, controlling a speed development using a PID controller always amounts to responding. At a time t1, the actual value 12 is below the target value 10. This is due to the start-up behavior and the inertia of the system. Thus, at the time t1, a control variable 14 is above the target value 10. According to the development of the target value, a constant speed is to be maintained at the time t2 after an acceleration has come to an end. Since the PID controller can always only compare current values, an overshooting of the actual value curve occurs at the time t2. By a corresponding counter-control using the control variable 14, an undershooting of the target value 10 is caused at the time t3. Such over- and undershooting occurs until the system has settled. From a time t4, when a braking or a reduction of the speed is performed, corresponding curve progressions are obtained. Since the PID controller, as illustrated in FIG. 1, can always only respond to an instantaneous comparison between the actual value and the target value, no high quality of control can be achieved.
Another drawback of PID controllers is that PID controllers cannot respond to long-term changes such as signs of wear, temperature variations or different qualities of the raw material. As a consequence, a PID-controller is unable to compensate, for example, signs of wear from a certain degree on. This compromises the quality of the parts produced.
EP 1 245 364, EP 1 163 993 and U.S. 2002/0163097 describe different control methods for injection molding machines, which, however, do not solve the above problem.